Optical fiber and optical transmission line and optical communication system including such optical fiber

ABSTRACT

Optical fibers to form an optical transmission line suitable for WDM transmission in a wide-spreading wavelength band, having the following characteristics and parameters: a dispersion in absolute value of 0.5 ps/nm/km to 9 ps/nm/km in a wavelength band of 1430 nm to 1625 nm, a dispersion slope in absolute value of 0.04 ps/nm 2 /km or less at a wavelength of 1550 nm, a mode field diameter of 7 μm or less at a wavelength of 1550 nm and a cable cutoff wavelength of less than 1430 nm; core  11  surrounded by cladding  7 , core  11  being at least two-layered (first layer  1  at the center and second layer  2  surrounding the first layer; relative refractive index of glass layer Δ 1  with reference to the cladding being adjusted to not less than 0.6 but not more than 1.6%, relative refractive index of second layer Δ 2  with reference to the cladding being adjusted to a negative value.

FIELD OF THE INVENTION

The present invention relates to optical fiber suitable for wavelengthdivision multiplexing (WDM) transmission, and optical transmission line,and optical communications system using such optical fibers.

BACKGROUND OF THE INVENTION

The volume of communication information data tends to greatly increasewith the advancement of the information society necessitating thewidespread use of wavelength division multiplexing (WDM) transmission inthe field of telecommunications. WDM transmission enables to transmitoptical signals of plurality of wavelengths through a single opticalfiber.

The erbium doped fiber amplifier (EDFA) is today developed and appliedto amplify the optical signals at points of relay in a WDM transmissionsystem. The EDFA does not require a process of transforming opticalsignals into electric signals at each wavelength, accelerating thespreading of WDM transmission worldwide.

In the meantime, Raman amplifiers with the Raman amplification are ofgreat interest as a new optical amplifier. The Raman amplification is asystem which amplifies light using the stimulated emission in Ramanscattering. The amplification efficiency depends largely on thenon-linearity of an optical fiber. As usual, the more the non-linearityis enhanced, the more the efficiency is improved.

FIG. 23 is a diagram for a example of optical transmission systemapplying the Raman amplification. The output of the Signal Light Sources4 al to 4 an have different wavelengths of signal lights and aremultiplexed by Optical Multiplexer 15.

The output of the Pump Light Sources 3 al to 3 an have differentwavelengths of pump lights, and are in a multimode lasing. The pumplights from Pump Light Sources 3 al to 3 an are mixed by OpticalMultiplexer 16. The pump lights and signal lights (WDM signal lights)are multiplexed by Optical Multiplexer 10 and led into Optical Fiber 8in the optical transmission line.

The WDM signal lights in Optical Fiber 8 propagate as being Ramanamplified till the receiving end, and then are demultiplexed for therespective wavelengths by Optical Demultiplexer 9, and received byOptical Receivers 7 al to 7 an.

FIG. 23 is an example of co-propagation pumping system in which the pumplights for the Raman amplification propagate in the same direction asthe transmission signal light in an optical fiber (Optical Fiber 8). Bycontrast, a second example, as shown in FIG. 24, is a counterpropagation pumping system in which pump lights propagate in the reversedirection to the transmission signal lights in an optical fiber (OpticalFiber 8).

Light Sources 13 al to 13 an in FIG. 24 output pump lights, which aremultiplexed by Optical Multiplexer 26. The pump lights were coupled tothe Optical Multiplexer 20 to propagate in the reverse direction tosignal lights.

Moreover, a third example shown in FIG. 25 is a bi-directional pumpingsystem in which pump lights propagate in both directions in an opticalfiber (Optical Fiber 8).

Concurrently in the same diagram, the pump lights from Pump LightSources 3 al to 3 an in FIG. 25 are multiplexed by Optical Multiplexer16, and those from Pump Light Sources 13 al to 13 an are multiplexed byOptical Multiplexer 26. The pump lights propagating in the same andreverse direction to the transmission signals are multiplexed withtransmission signals by Optical Multiplexer 10 and 20, respectively andthen fed into Optical Fiber 8. It is preferable to apply abi-directional pumping for making the light intensity longitudinallymore uniform throughout the Optical Fiber 8.

If Optical Fiber 8 is made of silica, the peak gain for the Ramanamplification appears at a frequency 13 THz lower than the frequency ofthe pump light source (wavelength about 100-110 nm longer). In brief, inthe Raman amplification there are amplified signal lights at a 100-110nm long wavelength from the pump light wavelengths.

Consequently, as for an optical transmission system in a wavelength bandof 1.5 μm, for instance, a maximum Raman gain can be obtained for signallights at 1580 nm when input pump light is at 1480 nm.

The optical fiber transmission characteristics depend on thetransmission loss and chromatic dispersion. A single mode fiber offers asingle mode of signal propagation as free from mode dispersion, andaccordingly its transmission band is limited by material dispersion andwaveguide dispersion. Conventional single mode fibers have a zerodispersion at a wavelength of 1.3 μm band (hereinafter called at 1.3 μm)or thereabout, and a lowest transmission loss in a wavelength band of1.5 μm (hereinafter called in 1.5 μm). Against this backdrop, a plan forDispersion Shifted Fiber (DSF) was proposed in an attempt to shift azero dispersion from 1.3 μm band to 1.5 μm band.

It is difficult to shift the zero dispersion from 1.3 μm to 1.5 μm bychanging the material dispersion of an optical fiber, and is obtained bychanging the wave guide dispersion. In short, an refractive indexprofile adjustment to the core and cladding resulted in shifting thezero dispersion to 1.5 μm.

The optical fiber transmission characteristics can be more improvedgenerally by further reducing chromatic dispersion. Yet, launching WDMsignals into an optical fiber with too low chromatic dispersion inducesunwanted waves in consequence of Four Wave Mixing (FWM), leading into afault of the increase in the inter-channel cross talk. This fault wassolved by the proposal of Non-Zero Dispersion Shifted Fiber (NZDSF).

One of the research and development tasks for the WDM transmission is topursue a broader transmission band of wavelength. The main range ofoperating wavelengths has been made up of the C-band (1530-1565 nm) andthe L-band (1565-1625 nm). Recently, a reduced dispersion slope NZ-DSFto cover up-to the S-band (1460-1530 nm) is substantiated and proposedin papers at “ECOC '01 PD A1-5 (2001), OECC '02 11D1-2 (2002)”.

However, even a reduced dispersion slope NZ-DSF is subject tolimitations on its shorter-wavelength range by the zero dispersionwavelength, and on the longer wavelength range by the cumulativedispersion Accordingly, the wavelength band for transmission was limitedto around 200 nm. In addition, the zero-dispersion wavelength of theNZ-DSF limits pump light wavelength band for the Raman amplification,almost unworkable in the S-band.

One purpose of the present invention is to provide optical fiberssuitable for the WDM transmission in a broad wavelength range betweenthe E-band and U-band, also to allow for Distributed Raman Amplificationin the S-band and to provide optical fibers with lowered bending lossand suppressed non-linearity

SUMMARY OF THE INVENTION

As shown carve “a” in FIG. 26, it is known that four wave mixing (FWM)F_(1-P) arises around signal lights (S_(1-n)) in the case azero-dispersion wavelength of an optical fiber interposes betweenwavelengths of signal lights (S_(1-n)) and pump lights (R_(1-m)). Inaddition, formula F_(fwm)=f_(i)+f_(j)−f_(k) holds, where F_(fwm) denotesa FWM frequency, f_(i), f_(j) and f_(k) denote frequencies of signallights (S_(1-n)) and pump lights (R_(1-m)), given i≠k, i≠j.

Also it is known that the intensities of FWM light F_(1-P) are inproportion to the generation efficiency “η” am as shown in Formula 1, aspublished, for instance, in “Lightwave Technology, vol. 8, No. 9, pp.1402 to 1990 (by Mari W. Maeda J.)”.

$\begin{matrix}{\eta = {\frac{\alpha^{2}}{\alpha^{2} + {\Delta\beta}^{2}}\left( {1 + \frac{4{\mathbb{e}}^{{- \alpha}\; L}{\sin^{2}\left( \frac{{\Delta\beta}\; L}{2} \right)}}{\left( {1 - {\mathbb{e}}^{{- \alpha}\; L}} \right)^{2}}} \right)}} & (1)\end{matrix}$

where α denotes a transmission loss, L denotes a fiber length, and Δ βdenotes a setting for phase-matching in FWM (see Formula 2).

$\begin{matrix}\begin{matrix}{{\Delta\beta} = {\beta_{i} + \beta_{i} - \beta_{k} - \beta_{fwm}}} \\{\cong {\frac{{\pi\lambda}^{4}}{c^{z}}{Ds}\left\{ {\left( {f_{i} - f_{0}} \right) + \left( {f_{j} - f_{0}} \right)} \right\}\left( {f_{t} - f_{k}} \right)\left( {f_{j} - f_{k}} \right)}}\end{matrix} & (2)\end{matrix}$

where β is a propagation constant, c is the light velocity, λ iswavelength, Ds is a dispersion slope in optical fiber, f_(i), f_(j) andf_(k) are frequencies of signal lights S_(1-n). and pumping lightsR_(1-m), f₀ is a frequency converted from the zero-dispersion wavelengthof an optical fiber. Notably, the FWM action prominently emerges in anoptical fiber where signal and pumping lights propagate in the samedirection. Formula 2 can be approximated in the case of signal andpumping lights propagating in the same direction through optical fiber8, as shown in the above Formula 2.

According to Formulae 1 and 2, where a zero-dispersion wavelength ofoptical fiber interposes between signal lights S_(1-n) and pumpinglights R_(1-m), the FWM (F_(1-p)) generation efficiency between thesignal and pumping lights increases.

As described above, where the FWM F_(1-p) occurs to degrade thetransmission performance of signal lights S_(1-n), the increased FWMgeneration efficiency η results in an increase in the intensity of thepumping light absorbed by the FWM, thereby disallowing a large Ramangain for the signal lights.

In addition, since the FWM generation efficiency between signal lightsand pumping lights becomes larger when both, the lights propagate in thesame direction, making the bi-directional piping not available, theoptimal optical transmission system cannot be built up.

In conclusion, the suppression of the FWM in an optical fiber is one ofthe significant tasks to realize a WDM Raman transmission system.

For reference, the WDM transmission is implemented, for instance, byusing an erbium-doped fiber amplifier mainly in the C band between 1530nm and 1565 nm, while an expansion of the WDM transmission band to 1530nm and 1625 nm, notably, in the so called L-band of 1565 nm to 1625 nm,is under study.

A Raman WDM transmission line covering the C and L bands needs pumplight at a wavelength around 100 nm shorter than the shortest wavelengthof the transmission wavelength band. The FWM in the optical fibers isconceivably suppressed by a zero-dispersion wavelength outside of therange of 1430 nm, 100 nm shorter than 1530 nm, to 1625 nm.

Also, it is earnestly desired to enable WDM transmission in the S bandbetween 1460 nm and 1530 nm, and in order to use the S, C and L bandsfor the WDM transmission, it is preferable for a zero-dispersionwavelength to be outside of the range of 1360 nm to 1625 nm.

Also, the Raman WDM transmission system requires making optical fibercharacteristics or parametric values, namely, dispersion slope,effective area (Aeff), transmission loss, absorption loss caused byhydroxyl ion, polarization mode dispersion (PMD) etc, appropriate, inaddition to the above FWM suppression.

For example, a large dispersion slope results in waveform distortion dueto dispersion, and a large effective area leads to an insufficient Ramanamplification efficiency. Similarly, a high transmission loss limits thetransmission in a long distance, and high polarization mode dispersionbrings in a long delay between signal lights due to the polarized lightdirection especially in high-speed transmission, degrading the signallight transmission.

Moreover, absorption loss caused by hydroxyl ions adversely affectsbroadening the WDM band because of the insufficient Raman amplificationefficiency in the case the pump lights wavelength band include 1385 nmor thereabout.

Up to the present, no optical fiber of the above requirements has beenproposed, and no high-quality and broadband Raman amplification systemhas been built up.

For instance, a conventional single mode optical fiber (SMF) has nozero-dispersion wavelength between 1360 nm and 1625 nm and hence allowsfor the suppression of the FWM. But, SMFs have relatively low values ofn₂/A_(eff) which prevents to create a Raman gain enough to compensatethe loss of the transmission line. (Note n₂: Karr coefficient andA_(eff): effective area).

For instance, as shown in FIG. 27, the n₂/A_(eff) of 4.4×10⁻¹⁰/W orlarger is required to make the fiber loss and Raman gain equal. But,SMFs cannot meet this requirement (see the slant-lined zone).

In addition, the SMF has a dispersion of approximately +17 ps/nm/km in awavelength band of 1.5 μm. As a result, transmission signals at the 1.5μm band may cause the intra-channel four-wave mixing by pulse broadeningdue to such large dispersion. Therefore, the SMF is undesirable for aWDM transmission system applying the Raman amplification.

For example, FIG. 28 presents absolute-dispersion value vs. pulsebroadening due to dispersion. (Note T: inversion of a bit rate, t: FWMfrequency of pulses). With a t/T value of 0.4 or less, the intra-channelfour wave mixing can be suppressed.

As described in the above, for transmission systems with 10 Gbps ormore, an absolute dispersion of the optical fiber should be 9 ps/nm/kmor less. But, the SMF as shown in the slant-lined range, has adispersion of approximately 17 ps/nm/km and exceeds a t/T value of 0.4,and therefore is not desirable for an optical transmission line intransmission systems with 10 Gbps or more.

Similarly it is known that, considering the simultaneous transmission ofsignal light in both the C and L bands, the difference in dispersionbetween both bands can be retardation for broadening the WDMtransmission band, even if the reduced dispersion slope NZ-DSF would beapplied Also, taking into consideration of the advanced development ofRaman amplifiers, the NZ-DSF can conceivably be applied to combine theS, C and L-bands for simultaneous signal transmission. Yet, the opticalfibers disclosed at “ECOC '01 PD A1-5 (2001)” and “OECC '0211D1-2(2002)” involve a zero-dispersion wavelength in the Raman pumplights band for S-band of 1360 nm to 1430 nm, consequently FWM arises tomake the Raman amplification unavailable. Their alternative fiber isneeded to solve the limitation.

The object of the present invention is to provide an optical fiber withsuppressed FWM, lowered dispersion, reduced dispersion slope absolutevalue, efficient Raman amplification and preferably lowered transmissionloss and polarization mode dispersion, suitable for WDM transmissionwith the Raman amplification, and optical transmission line and opticaltransmission system.

In order to attain the above-mentioned purpose, the following opticalfibers with the characteristics mentioned therein are proposed.

-   1) An optical fiber characterized by:    -   an absolute value of dispersion of not less than 0.5 ps/nm and        not more than 9 ps/nm/k over a wavelength range of 1430 nm to        1625 nm,    -   an absolute value of dispersion slope of 0.04 ps/nm²/km or less        at 1550 nm,    -   a mode field diameter (MFD) of 7 μm or smaller at 1550 nm, and    -   a cable cutoff wavelength of 1430 nm or shorter-   2) An optical fiber in which the dispersion “D” is not less than 2    ps/nm/km and not more than 8 ps/nm/km: (2≦D≦8 ps/nm/km) over a    wavelength range of 1400 to 1700 nm, and at least one extreme value    of dispersion within said wavelength range.-   3) An optical fiber having    -   a dispersion of 4 ps/nm or more r at 1550 nm,    -   a positive dispersion slope of not more than 0.050 ps/nm²/km at        least at a predetermined wavelength within a range of 1460 nm to        1625 nm,    -   a cable cutoff wavelength of 1550 nm or shorter at a length of 2        m,    -   a zero-dispersion wavelength of 1460 nm or shorter, and    -   a transmission loss of 1.5 dB/km or lower at 1385 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a refractive index profile, FIG. 1( a), and across-section, FIG. 1( b), of an optical fiber according-to the firstembodiment of the present invention

FIG. 2 shows a sample correlation in wavelength of pumping lights andsignal lights, along with a zero-dispersion wavelength in the firstembodiment according to the present invention. FIG. 2( a) shows a samplecorrelation in the case of a zero-dispersion wavelength being 1360 nm orshorter and FIG. 2( b) shows a sample correlation in the case of azero-dispersion wavelength being 1430 nm or shorter.

FIG. 3 shows a sample correlation in wavelength of plumping lights andsignal lights, along with a zero-dispersion wavelength in the case of azero-dispersion wavelength being 1625 nm or longer, in the firstembodiment according to the present invention.

FIG. 4 draws wavelength characteristics of the chromatic dispersion foroptical fibers in the first embodiment according to the presentinvention.

FIG. 5 draws a wavelength characteristic of the chromatic dispersion foran optical transmission line in the first example of the firstembodiment, according to the present invention.

FIG. 6 draws a wavelength characteristic of the chromatic dispersion foran optical transmission line in the second example of the firstembodiment, according to the present invention.

FIG. 7 shows a refractive index profile FIG. 7( a) and a cross-sectionFIG. 7( b) of an optical fiber in the second embodiment (four-layered),according to the present invention.

FIG. 8 shows a refractive index profile FIG. 8( a) and a cross-sectionFIG. 8( b) of an optical fiber in the second embodiment (five-layered),according to the present invention.

FIG. 9 draws wave length characteristics of the chromatic dispersion ofthe optical fibers in the second embodiment according to the presentinvention.

FIG. 10 shows a refractive index profile FIG. 10( a) and a cross-sectionFIG. 10( b) of an optical fiber in the third embodiment according to thepresent invention.

FIG. 11 draws simulated chromatic dispersion characteristics of the ofthe optical fibers in the third embodiment according to the presentinvention.

FIG. 12 is a graph adds a Raman amplification band (1360-1525 nm) andwavelength characteristics of the chromatic dispersion for an opticalfiber with a dispersion of 2 to 8 ps/nm/km in the same band to FIG. 11.

FIG. 13 draws chromatic dispersion characteristics of for the opticalfibers in the third embodiment according to the present invention.

FIG. 14 is a diagram of an optical transmission system with opticalfibers according to the present invention.

FIG. 15 is a diagram of another optical transmission system with opticalfibers according to the present invention.

FIG. 16 is a diagram of more other optical transmission system withoptical fibers according to the present invention.

FIG. 17 shows a refractive index profile FIG. 17( a) and a cross-sectionFIG. 17( b) for an optical fiber in the fourth embodiment according tothe present invention.

FIG. 18 draws transmission loss difference against transmission loss at1385 nm in the fourth embodiment according to the present invention.

FIG. 19 draws wavelength dependence of transmission loss againstdifference of transmission loss at 1385 nm in the fourth embodimentaccording to the present invention. FIG. 19( a) shows a sample in thecase of transmission loss at 1385 nm is 1.5 dB/km or lower and FIG. 19(b) shows a sample in the case of transmission loss at 1385 nm is 1.5dB/km or higher.

FIG. 20 draws transmission loss at 1385 nm vs. etching thickness ofpreform surface (HF: hydrofluoric acid) in the fourth embodimentaccording to the present invention.

FIG. 21 draws wavelength characteristics of the chromatic dispersion forthe optical fibers according to the present invention in the fourthembodiment according to the present invention and with a steepdispersion slope.

FIG. 22( a)-(c) draw wavelength characteristics of the chromaticdispersion for the optical in the fourth embodiment according to thepresent invention.

FIG. 23 is a diagram for an example copumping Raman amplification WDMtransmission system.

FIG. 24 is a diagram for an example counter-pumping Raman amplificationWDM transmission system.

FIG. 25 is a diagram for an example bi-directional pumping Ramanamplification WDM transmission system.

FIG. 26 shows a conventional sample correlation in wavelength of pipinglights and signal lights, along with a zero-dispersion wavelength.

FIG. 27 draws input/output signal intensity vs. n₂/A_(eff) for anoptical fiber.

FIG. 28 draws pulse spread vs. chromatic dispersion (absolute value).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the preferred embodiments of this invention are describedwith reference to the drawings.

Embodiment 1

FIG. 1( a) shows a refractive index profile and FIG. 1( b) shows across-section of an optical fiber according to the first embodiment ofthe present invention. The refractive index profile in FIG. 1( a)features to be relatively simple and easy to design and control,compared to the other profiles.

Optical fiber of the first embodiment comprises of a core 11 at thecenter and a cladding 7 surrounding the core 11. In addition, the corehas at least two layers with a first layer 1 at the center and a secondlayer 2 surrounding the first layer 1; the first layer 1 has a relativerefractive index difference Δ1 of not less than 0.6% and not more than1.6% with the cladding 7; the second layer has a relative refractiveindex difference Δ2 of negative value with the cladding 7; a diameter ofthe first layer 1 is shown as “a” and a diameter of the second layer 2is shown as “b”.

The present patent specification defines the above relative refractiveindex differences (Δ1, Δ2, Δn) in the following approximation formulas(Formula 3, Formula 4, Formula 5):Δ1(%)={(n ₁ ² −n _(c) ²)/2·n _(c) ²}·100  (3)Δ2(%)={(n ₂ ² −n _(c) ²)/2·n _(c) ²}·100  (4)Δn(%)={(n _(n) ² −n _(c) ²)/2·n _(c) ²}·100  (5)(n₁: refractive index of first layer, n₂: refractive index of secondlayer n_(n): refractive index of nth layer, n_(c): refractive index ofcladding)

Table 1 presents trial optical fibers #1-1 to #1-14 in the firstembodiment with parametric values and performance characteristics,including relative refractive index differences for Δ1 and Δ2, in theunit of %. In addition, Δ1 (Ge) and Δ1 (F) show contributions of dopedGermanium and Fluorine, respectively.

TABLE 1 Transmission Dispersion Dispersion loss MFD Bending cutoffps/km/nm slope dB/km μm loss wave- Δ1 Δ1 Δ1 b 1360 1430 1625 ps/km/nm1390 1550 1560 PMD nz/Aeff dB/m length (Ge) % (F) % Total % Δ2 % a μm μmnm nm nm 1560 nm nm nm nm pa/√km 10⁻¹⁰/w 1550 nm nm #1-1 0.60 — 0.60−0.45 6.1 14.1 0.51 4.13 6.92 0.009 0.612 0.202 6.3 0.19 8.6 4.8 890#1-2 0.60 — 0.60 −0.30 6.4 13.9 −0.17 2.79 8.57 0.026 0.651 0.206 6.790.08 7.4 3.7 916 #1-3 0.70 — 0.70 −0.30 4.9 15.8 −3.04 0.88 7.96 0.0330.668 0.208 6.08 0.11 9.9 2.2 935 #1-4 0.70 — 0.70 −0.40 5.7 11.9 −0.523.05 8.29 0.023 0.664 0.211 6.11 0.15 9.8 1.1 982 #1-5 0.70 — 0.70 −0.405.2 12.1 −5.46 −3.01 −5.89 −0.002 0.697 0.207 5.99 0.13 10.1 4.3 870#1-6 0.90 — 0.90 −0.40 4.4 11.7 −8.73 −6.10 −5.74 −0.003 0.717 0.2225.40 0.42 14.0 1.6 896 #1-7 1.00 — 1.00 −0.40 4.4 11.8 −8.90 −6.03 −3.380.009 0.728 0.234 6.19 0.35 16.0 0.7 926 #1-8 0.80 — 0.80 −0.40 4.9 11.9−5.98 −3.20 −2.37 −0.001 0.674 0.211 5.68 0.04 12.0 3.2 820 #1-9 1.20 —1.20 −0.55 6.0 12.4 −3.31 0.59 7.53 0.032 0.810 0.235 4.90 0.03 19.8 0.51010 #1-10 1.40 — 1.40 −0.55 4.6 10.9 −2.42 1.31 7.89 0.030 0.880 0.2424.60 0.15 24.5 0.2 1118 #1-11 1.60 — 1.60 −0.65 4.7 11.1 −2.79 1.11 8.320.034 0.950 0.247 4.43 0.18 28.7 0.1 1166 #1-12 1.10 −0.40 0.70 −0.405.8 11.9 −0.60 3.12 8.43 0.024 0.490 0.212 6.01 0.19 12.5 0.9 996 #1-131.05 −0.45 0.60 −0.45 6.0 14.0 0.52 4.14 7.06 0.009 0.480 0.215 6.270.22 11.2 4.7 902 #1-14 1.05 −0.45 0.60 −0.45 6.1 14.1 0.53 4.08 7.110.009 0.450 0.216 6.29 0.18 11.2 4.9 889

In addition, “a” and “b” in Table 1 denote a diameter of the first layerand a diameter of the second layer, respectively, and each “dispersion”refers to the values at the wavelength underneath. MFD refers to themode field diameter at 1550 nm; PMD represents polarization modedispersion at 1550 nm; “bending loss” refers to the values in a bendingdiameter of 20 mm at 1550 nm; “cutoff wavelength” refers to the cablecutoff wavelength, namely the cutoff wavelength at a fiber length of 22m.

The first embodiment apply silica as the host glass to produce opticalfibers; consequently, silica glass forms those optical fibers. Anoptical fiber of the embodiment has its first layer doped at least withgermanium and the second layer at least with fluorine. The trial opticalfibers #1-1 to #1-11 in Table 1 were fabricated according to thisexample of composition.

Meanwhile, another optical fiber of the embodiment has its first layerdoped at least with germanium and fluorine, and the second layer atleast with fluorine. The trial optical fibers #1-12 to 1-14 in Table 1were fabricated according to this example of composition; notably, theirfirst and second layers were doped with substantially the same amountsof fluorine.

All optical fibers of the first embodiment made a transmission loss of0.25 dB or less at 1550 nm.

Considering a long distance, e.g., 80 km of transmission line, thetransmission loss requires to be 0.25 dB/km or less, if a total loss of20 dB or less is targeted. The Raman amplification can offset a certainloss increase, but it is necessary to make the loss 0.25 dB/km or lessin a signal light band, if generating of noise etc. is taken intoconsideration.

Thus, an optical fiber of the first embodiment, which made atransmission loss of 0.25 dB/m or less at 1550 nm, is suitable for longdistance transmission in a 1.55 μm band, as described above.

An optical fiber of the first embodiment made a transmission loss of 1.0dB/km or less at 1385 nm.

As generally known, the optical fiber has absorption loss arising fromhydroxyl ion at 1385 nm or thereabout, which will not relatively affectthe C-band operation, but may adversely affect the S-band operation.

That is, when applying an optical fiber to the optical transmission linefor the Raman amplification in the S band, absorption loss result inloss of the pump light since both wavelength bands overlap around thewavelength of 1385 nm.

Optical fibers with a loss of over 1 dB/km at 1385 nm cause large pumplight loss and require an expensive light source and large electricpower, and consequently result in a problem in cost.

An optical fiber of the first embodiment, which made a transmission lossof 1.0 dB/km or less at 1385 nm, can suppress the pump light loss in anoptical transmission line equipped with Raman amplifiers in the S-band.

An optical fiber of the embodiment made a loss increase of 10% or lessat 1385 nm under a hydrogen atmosphere; here, hydrogen examinationconditions follow IEC60793-2 Amendment 1,2001-08 Annex B. In thisspecification, the loss increase under hydrogen atmosphere is defined asmentioned in the above.

Conventionally, a transmission system fails by a loss increase of 10% ormore at 1385 nm due to the hydrogen generation in an optical fiber cableunless the input power is not increased.

Since an optical fiber of the first embodiment made loss increase of 10%or less at 1385 nm under a hydrogen atmosphere, it can avoid thisproblem.

All the trial optical fibers #1-1 to #1-14 have a polarization modedispersion (PMD) of 0.5 ps/√{square root over ( )}km or less at 1550 nm,each.

As generally known, the polarization mode dispersion is one of thelimitations to prevent high speed transmission and needs to be not morethan 0.5 ps/√{square root over ( )}km. Otherwise, high speedtransmission is impossible without compensating the polarization modedispersion.

All optical fibers of the first embodiment, made not more than 0.5ps/√{square root over ( )}km in polarization-mode dispersion asdescribed, are enabled to perform in high speed transmission, withoutcompensating the polarization mode dispersion.

In particular, trial optical fibers #1-1 to #1-5, #1-8 to #1-12 and#1-14 in Table 1, in which each made a polarization mode dispersion ofnot more than 0.2 ps/√{square root over ( )}km, can suppress theundesirable effect of polarization mode dispersion even more, and canperform in high speed transmission at some 40 GB/s, even withoutcompensating the polarization mode dispersion.

Also, trial optical fibers #1-1, #1-13, and #1-14 in Table 1 have apositive dispersion throughout a wavelength band of 1360 nm to 1625 nm,an absolute value of chromatic dispersion of not less than 0.5 ps/nm/kmand not more than 9 ps/nm/km, and non zero dispersion wavelengththroughout said wavelength band.

In short, these trial optical fibers have a zero dispersion wavelengthshorter than the shortest wavelength R₁ (here 1360 nm) of the pump lightR_(1-m) as shown in FIG. 2( a).

Also, trial optical fibers #1-5 to #1-8 in Table 1 have a negativedispersion throughout a wavelength band of 1360 nm to 1625 nm, anabsolute value of chromatic dispersion of not less than 0.5 ps/nm/km andnot more than 9 ps/nm/km, and non zero dispersion wavelength throughoutsaid wavelength band.

In short, these trial optical fibers have a zero dispersion wavelengthlonger than the longest wavelength R_(n) (here 1625 nm) of the signallight R₁ to R_(m) as shown in FIG. 3.

As an example of representation of these optical fibers, the wavelengthcharacteristics of the chromatic dispersion of the optical fibers of#1-1 and #1-5 are shown in FIG. 4. The characteristic line “a” shows thewavelength characteristics of the chromatic dispersion ofabove-mentioned #1-1, and the characteristic line “b” shows thewavelength characteristics of the chromatic dispersion ofabove-mentioned #1-5.

As these fibers have a zero-dispersion wavelength over the range of 1360nm to 1625 nm, FWM generation can be suppressed throughout a broadwavelength band, from S-band to L-band (1460 nm to 1625 nm), and therebyWDM transmission with the Raman amplification can be performedefficiently.

Also, trial optical fibers #1-2 to #1-4 and #1-9 to #1-12 in Table 1have an absolute value of chromatic dispersion of not less than 0.5ps/nm/km and not more than 9 ps/nm/km, and non zero dispersionwavelength throughout a wavelength band of 1430 nm to 1625 nm.

In short, these trial optical fibers have a zero dispersion wavelengthshorter than the shortest wavelength R₁ (here 1430 nm) of the pumplights R₁ to R_(m) as shown in FIG. 2( b).

In short, in these fibers, FWM generation can be suppressed throughout abroad wavelength band, from C-band to L-band, and thereby WDMtransmission with the Raman amplification can be performed efficiently.

All optical fibers of the first embodiment have an absolute value ofdispersion of 9 ps/nm/km or less throughout a wavelength band of 1360 nmto 1625 nm, thereby suppressing pulse broadening, and reducingintra-signal non-linear effect.

In a high speed WDM transmission at 40 Gb/s which is applied to theRaman amplification, a cumulative dispersion of 60 ps/nm or more causessignificant waveform distortion, and makes dispersion compensation anecessity, even if each optical fiber in the transmission line does nothave so large chromatic dispersion.

Broad band transmission is preferably designed so that the dispersion inthe broad band can be compensated with a single dispersion compensator.But, even the dispersion compensating fibers, with an advantage of broadband application, can hardly have a large dispersion slope, andtherefore it is difficult to reduce the dispersion to dispersion sloperatio (DPS: dispersion/dispersion slope) to 200 nm or less.

Therefore, in the case of an optical fiber having an absolute value ofdispersion of 8 to 9 ps/nm/km to be used in a transmission line, forinstance, it is desired to have an absolute value of dispersion slope of0.04 ps/nm²/km and the same sign as the dispersion. In addition, in theease of an optical fiber having an absolute value of dispersion of 4 to6 ps/nm/km to be used in a transmission line, it is desired to have anabsolute value of dispersion slope of 0.03 ps/nm²/km and the same signas the dispersion.

All optical fibers of the first embodiment, have an absolute value ofdispersion slope of 0.04 ps/nm²/km or less at 1550 nm, and satisfy theabove requirements. As a result, dispersion changes can be suppressed inthese fibers, and thereby can make dispersion compensations easy.

The effective area (Aeff) and the mode field diameter (MFD) arecorresponding to each other in a unique theorem. An extremely largeeffective area results in an insufficient Raman amplificationefficiency. For reference, the dispersion shifted fiber (DSF) in ITU-Trecommendation G.653 has a mode field diameter of 7.8 to 8.5 μm, whichcorresponds to a minimum effective area of around 48 μm².

A Raman-amplification gain in stark contrast with that with thereferenced DSF can preferably be realized with an effective area of 40μm² or less (corresponding to 7 μm or less in mode field diameter).

All optical fibers of the first embodiment, having a mode field diameterof 7 μm or less at 1550 nm, can be more suitable for the efficient Ramanamplification, than the referenced DSF, thereby enabling desirable WDMtransmission.

Also all optical fibers of the first embodiment, having a cable cutoffwavelength of 1360 nm as described above, can satisfy the requirement ofthe single-mode propagation for pumping lights and signal lights.

Furthermore, all the trials are with a bending loss of 5 dB/m in abending diameter of 20 mm, at 1550 nm, and therefore as generally knowncan realize optical fiber cables for use with low transmission loss overa broad wavelength band of the S-band to the L-band.

The present inventor has made various studies to formulate refractiveindex profiles for the optical fibers of the embodiment. As a result, itturns out that a negative value for relative refractive index differenceΔ2 will lead up to a rather reduced absolute value of dispersion slopein an optical fiber, as shown in FIG. 1.

As for relative refractive index difference Δ1, a relative refractiveindex difference of less than 0.6% makes the mode field diameter of 7 μmor less at 1550 nm and a relative refractive index difference of morethan 1.6% makes the transmission loss of 0.25 dB/km or more at 1550 nm.

According to the above, trial optical fibers were fabricated (see Table1, for example), with their profiles formulated, targeting on a relativerefractive index difference Δ1 of not less than 0.6% and not more than1.6%, and a relative refractive index difference Δ2 of negative value.

The present inventor has made various studies of manufacturing method ofoptical fibers, and decided the following methods as preferablemanufacturing methods for an optical fiber according to the firstembodiment:

The first method is characterized that the first layer 1 and the secondlayer 2 of the core are formed in one process. The second method ischaracterized that the cladding region whose diameter is more than twotimes that of the second layer of the core, see FIG. 1( b); Zone CL asbroken-lined, is formed in the same process along with the first layer 1and the second layer 2.

Preferable methods are applied to trials #1-13 and #1-14 in Table 1.Trial 13 is manufactured with the first method, and trial 14 with thesecond.

All optical fibers according to the first embodiment have the samerefractive index profile as shown FIG. 1. Yet notably, trial #1-1 hadits first layer and second layer formed in two different processes.Trial #1-13 had its first and second layers formed in one process. Intrial #1-14, the first and second layers and a cladding region, whosediameter is more than two times that of said second layer 2, were formedin one process.

The method for trials #1-13 and #1-14 create no boundary faces betweenlayers in cores, whereby the loss due to hydroxyl ion can be suppressed;the transmission loss at 1385 nm can be reduced. For information, thetransmission loss at 1385 nm of trial #1-1 can seldom cause any problem.However, it is desirable to target on those values obtained in trials#1-13 and #1-14.

In addition, the transmission loss at 1385 nm of trial optical fiber#1-14 is even less, because the first and second layers and a part ofthe cladding were formed in the same process.

Moreover, optical fibers of trial #1-1 and trial #1-14 have the samemode field diameter, but differ in n₂/A_(eff). Optical fiber of Trial#1-14 had its first layer doped with both germanium and fluorine, and agreater doping concentration and a larger n₂ value than in trial #1-1,of which first layer has the same refractive index. As a result, trial#1-14 can have an n₂/A_(eff) value larger than trial 1, creating agreater Raman gain.

Now, an explanation about a first example of an optical transmissionline according to the first embodiment is given. The first exampletransmission line involved trial optical fibers #1-1 and #1-7 asconnected in series. Trial fiber #1-1 was positioned at the transmitterside and trial #1-7 at the receiver side. The ratio of lengths of #1-1to #1-7 was 1.6:1.

From the aspect of cumulative dispersion, at least one optical fiberwith positive dispersion and at least one optical fiber with negativedispersion are preferably applied to form an optical transmission line;this intention is reflected in the first example.

FIG. 5 shows the average dispersion curve for the optical transmission.Notably, the average dispersion over an optical transmission line cannever affect the pump light, and should be discussed with reference tothe transmission wavelength band between 1460 and 1625 nm, where theaverage dispersion is substantially zero over the wavelength band asshown in FIG. 5.

In the first example optical transmission line, the average dispersionin a predetermined wavelength within a wavelength band of 1460 to 1620nm (here throughout the band) is adjusted to not less than −1.0 ps/nm/kmand not more than 1.0 ps/nm/km. The average dispersion is −1 ps/nmkm at1460 nm and remains less than +1 ps/nmkm up to 1625 nm.

The first example optical transmission line with an absolute value ofdispersion of 1 ps/nm/km or less over the S, C and L-bands, realizes WDMtransmission and thereby there is suppressed waveform distortion due todispersion in all the above bands.

Therefore, the transmission line can be applied to form an opticaltransmission system with an average dispersion of not less than −1.0ps/nm/n and not more than +1.0 ps/nm/km in a predetermined wavelengthwithin a range of 1460 nm to 1625 nm (here throughout the range).

The optical transmission system is connected to an optical transmissionline with signal light sources and pump light sources which havedifferent wavelengths from the signal lights, and applicable forco-propagation pumping, counter propagation pumping and bi-directionalpropagation pumping system, shown as FIG. 23 to 25, for example.

Here, the determined wavelength band may be only the operatingwavelength band, unlike the example as shown. Thus, a partial or wholeoperating wavelength band can be made to have a dispersion in absolutevalue of 1 ps/nmkm or less, by a design to form an optical transmissionline.

For applying the Raman amplification of the counter propagation pumpingsystem to the dispersion managed transmission line, optical fiber havinga less Raman gain should not be positioned to the receiver side toacquire the required Raman gain. Accordingly, among the optical fibersto be laid out, optical fiber having the largest mode-field diametershould not be positioned next to the receiver side.

FIG. 6 shows the average dispersion for an optical transmission line inthe second example. The transmission line comprises of trial opticalfiber #1-1 and a dispersion compensating fiber of single-mode fiber, ata ratio of lengths of 12:1.

The dispersion compensating fiber for the optical transmission line inthe second example has a dispersion of −72 ps/nm/km at 1460 nm, −80ps/nm/km at 1550 nm and −94 ps/nm/km at 1625 nm.

An optical transmission line in the second example made an averagedispersion of not less than −1.0 ps/nm/km and not more than 1.0 ps/nm/kmin a predetermined wavelength within a range of 1460 nm to 1625 nm (herethroughout the range), and has the same effect as the one in the aboveexample. Similarly, an optical transmission system with an opticaltransmission line connected to the second example can also have the sameeffect as the one connected to an optical transmission line in the abovefirst example.

For reference the optical transmission line in the second example had+0.96 ps/nm/km of maximum dispersion at 1540 nm and −0.92 ps/nm/km ofminimum dispersion at 1626 nm.

Moreover, optical fibers for the transmission lines according to thefirst embodiment are not limited to the first and second example. Thatis, for instance, optical fibers according to the first embodiment mayhave the refractive index profiles other than the above mentioned, andalso the transmission line can involve at least one optical fiber withpositive dispersion connected with at least one optical fiber withnegative dispersion, depending upon the requirement.

The first embodiment allows a resultant optical transmission system withan average dispersion of not less than −1.0 ps/nm/km and not more than1.0 ps/nm/km in a predetermined wavelength band of 1460 to 1625 nm,where the band may be partial or throughout between both wavelengths.

In addition to the above case with an optical fiber and an opticaltransmission system connected to a Raman amplified WDM transmissionsystem, an optical fiber and an optical transmission system according tothe first embodiment can be applicable for a WDM transmission systemwith other than Raman amplifier, for instance, one with EDFA (ErbiumDoped Fiber Amplifier)

Embodiment 2

An optical fiber in a second embodiment has a zero dispersion wavelengthof 1350 nm or shorter, and makes it possible to suppress the FWM and thecumulative dispersion over a broad wavelength band of 1400 to 1700 nm.The conventional reduced dispersion slope NZ-DSF proposed until nowmakes it possible to realize a signal transmission over the S, C andL-bands (1460-1625 nm) The optical fiber according to the secondembodiment has a dispersion “D” of not less than 2 and not more than 8ps/nm/km (2≦D≦8 ps/nm/km) even in the longer wavelengths of U-band(1625-1675 nm) and the shorter wavelengths of the E-band (1365-1700 nm),along with the referenced three bands, suppressing the FWM generationand the cumulative dispersion.

In the case when optical fibers in the second embodiment are applied toa transmission line, a distributed Raman amplifier, and a discrete Ramanamplifier applied to at least one of the S, C and L-bands, can becombined to compensate the transmission loss over those bands (1460-1625nm). As the dispersion slope of the optical fiber is made to zero, thezero dispersion wavelength can be shifted to the shorter wavelength side(shorter than 1360 mm preferably 1340 nm) than in the reduced dispersionslope NZ-DSF, and thereby the FWM generation can be suppressed even inthe Raman amplification wavelength band for transmission signals in theS-band.

In addition, in the case when EDFA (Erbium Doped Fiber Amplifier), TDFA(Thulium Doped Fiber Amplifier), and a discrete Raman amplifier aresimultaneously used, an effective area (A_(eff)) of not less than 40 μm²(preferably not less than 45 μm²) at 1550 nm is made, considering thesuppression of the non-linear effect. Single mode propagation at leastin the C-band was ensured by adopting a cable cutoff wavelength of 1550nm or shorter. A bending loss in a bending diameter of 20 mm is made tobe 5 dB/m or less.

An optical transmission system is built up, by comprising an S-banddistributed Raman amplifier and the optical fibers having the abovedispersion as an optical transmission line. The optical transmissionsystem provides a high-quality WDM transmission. The optical fiber isespecially suitable for relatively short distance transmission such asmetropolitan network systems.

Hereinafter, the second embodiment of this invention is described withreference to the drawings

FIG. 7( a) and FIG. 8( a) show refractive index profiles and FIG. 7( b)and FIG. 8( b) show cross-sections of optical fibers according to thesecond embodiment of the present invention. Many kinds of the refractiveprofiles are available for the optical fibers Yet, the second embodimenthas a four or five-layered profile as shown in FIG. 7( a) and FIG. 8(a).

An optical fiber with a refractive index profile as in FIG. 7( a) has acore 11 at the center and a cladding 7 surrounding the core 11. Inaddition, the core has at least four layers (first 1, second 2, third 3,forth 4 from the center to outside), where any two adjacent layersdiffer in composition over each other. The layers are in a coaxiallayout. The cladding serves as the reference refractive index indicatorfor the four layers mentioned above.

In the optical fiber having four layered core according to the secondembodiment, the four relative refractive indices of the core are Chosensuch that Δ1>Δ3>Δ4>0>Δ2 (given Δ1 for the first layer, Δ2 for thesecond, Δ3 for the third, Δ4 for the forth; 0 for the cladding). Inaddition, the first layer has an α-profile.

FIG. 8( a) presents another refractive index profile. An optical fiberwith a refractive index profile as in FIG. 8( a) has a core 11 at thecenter and a cladding 7 surrounding the core 11. In addition, the corehas at least five layers (first 1, second 2, third 3, forth 4, fifth 5from the center to outside), where any two adjacent layers differ incomposition over each other. The layers are in a coaxial layout. Thecladding serves as the reference refractive index indicator for the fivelayers mentioned above.

In the optical fiber having five layered core according to the secondembodiment, the five relative refractive indices of the core are chosensuch that Δ1>Δ4>Δ5>0>Δ3>Δ2 (given Δ1 for the first layer, Δ2 for thesecond, Δ3 for the third, Δ4 for the forth, Δ5 for the fifth; 0 for thecladding). In addition, the first layer has an α-profile. A layer with anegative relative refractive index difference (lower refractive indexthan that of the cladding) is provided in addition to the four glasslayers in FIG. 7.

The second embodiment defines “a” is diameter of first layer, “b” isdiameter of second layer, “c” is diameter of third layer, “d” isdiameter of fourth layer, and “e” is diameter of fifth layer.

For the refractive index profile in FIG. 7( a) and FIG. 8( a), theoptimum parametric values were determined with the aid of a simulationprogram, for the relative refractive index differences Δ1, Δ2, Δ3, Δ4,and Δ5, α factor for the first layer, and for the diameters of eachlayers under the condition of a dispersion “D” of 2≦D≦8 ps/nm/km over arange of 1400 nm to 1700 nm, an effective area of 40 μm² or more at 1550nm, a bending loss of 5 dB/m or less at 1550 nm and a cutoff wavelengthof 1550 nm or shorter. The designed parameters are shown in #2-1, #2-2,and #2-3 in Table 2.

TABLE 2 Δ1 Δ2 Δ3 Δ4 Δ5 a:b:c:d:e Dc λ DP S Aeff λc BL % % % % % α a b cd e μm nm ps/nm/km ps/nm²/km μm² nm dB/m #2-1 0.48 −0.46 −0.05 0.4 0.1910 1 1.7 2.2 2.6 2.7 20.0 1500 6.58 0.0086 42.0 <1550 <1 1550 6.62−0.0067 44.6 2 #2-2 0.48 −0.46 −0.05 0.39 0.19 10 1 1.7 2.2 2.6 2.7 20.21500 7.17 0.0107 42.1 <1550 <1 1550 7.30 −0.0044 44.7 2 #2-3 0.5 −0.550.38 0.2 — 8 1 1.7 2.1 2.6 — 20.0 1500 5.42 0.0079 43.5 <1550 <1 15505.56 −0.0019 46.5 1 Dc: Core Diameter λ: Wavelength DS: Dispersion D:Dispersion slope Aeff: Effective area λc: Cable cut off wavelength BL:Bending loss

An optical fiber of the second embodiment has a dispersion “D” of 2≦D≦8ps/nm/km with at least one extreme value between 1400 nm and 1700 nm,making it comparatively easy to realize WDM transmission in principlewithout the aid of a dispersion compensation, and broad-bandamplification, including distributed Raman amplification in the S-band.

Optical fibers in the second embodiment are designed to have thefollowing parametric values and characteristic values: a dispersion “D”of −4≦D≦4 ps/nm/km at 1310 nm preferably −2≦D≦2 ps/nm/km at 1310 nm, acutoff wavelength of 1550 nm or shorter, a bending loss of 5 dB/m orless in a bending diameter of 20 mm at 1550 nm, an effective area of 40μm², a polarization mode dispersion of 0.1 ps √{square root over ( )}/kmor less at 1550 nm, and a zero-dispersion wavelength of 1350 nm orshorter.

An optical fiber with a four layered core as shown in FIG. 7( a) has afirst layer with a relative refractive index difference Δ1 of 0.3 to0.7% with reference to a cladding and a diameter of 7.0 to 10 μm. Thefabricated optical fiber has a diameter of 125 μm.

In addition, the first layer as shown in FIG. 7( a), shows a refractiveindex profile conforming to the following formula (6) with α of 4 ormore.n ²(r)=n1²{1−2·Δ1·(2r/a)^(α)}  (6)0≦r≦a/2

where n(r) expresses the refractive index at the radius “r”.

In addition, an optical fiber with a four layered core as shown in FIG.7( a) has, for instance, a refractive index profile as following: arelative refractive index difference of the second layer Δ2 with thecladding is −0.6 to −0.2%, a ratio of diameter of the second layer tothat of the first layer is 1.2 to 1.8, a relative refractive indexdifference of the third layer Δ3 with the cladding is 0.25 to 0.5%, aratio of diameter of the third layer to that of the first layer is 1.8to 2.2, a relative refractive index difference of the forth layer Δ4with the cladding is 0.05 to 0.2%, and a ratio of diameters of the forthlayer to the first layer is 2.0 to 2.7.

An optical fiber with a five layered core as shown in FIG. 8( a) has afirst layer at the center whose relative refractive index difference Δ1with reference to a cladding is 0.3 to 0.7%, and α of α-profile is 4 ormore. And, the diameter of the first layer is 6.5 to 10 μm whilefabricating an optical fiber with a diameter of 125 μm.

In addition, an optical fiber with a five layered core as shown in FIG.8( a) has, for instance, a refractive index profile as following: therelative refractive index difference of the second layer Δ2 with thecladding is −0.6 to −0.2%, a ratio of diameter of the second layer tothat of the first layer is 1.2 to 1.8, the relative refractive indexdifference of the third layer Δ3 with the cladding is −0.15 to −0.05%, aratio of diameter of the third layer to the first layer is 1.8 to 2.2,the relative refractive index difference of the forth layer Δ4 with thecladding is 0.25 to 0.65%, a ratio of diameter of the fourth layer tothat of the first layer is 2.0 to 2.7, the relative refractive indexdifference of the fifth layer Δ5 with the cladding is 0.05 to 0.50%, anda ratio of diameter of the fifth layer to that of the first layer is 2.2to 3.0.

An optical transmission line made up of optical fibers according to thesecond embodiment results in an optical transmission system with adistributed Raman amplifier and a discrete Raman amplifier in at leastone of the S, C and L-bands.

Optical fibers were made, according to the parametric values in Table 2,and turned out to have almost the designed dispersions. Samples #2-1 and#2-2 have five-layered core as shown in FIG. 8( a) and #3-3 has fourlayered core as shown in FIG. 7( a). Table 2 presents dispersions at1500 nm and 1550 nm, dispersion slopes at 1500 nm and 1550 nm, effectiveareas at 1500 nm and 1550 nm, cable cutoff wavelength (λc), bendinglosses in 20 mm diameter at 1500 nm and 1550 nm, zero dispersionwavelength λo, here core diameter is “d” as in FIG. 7, diameter is “e”as in FIG. 8. FIG. 9 shows the dispersion characteristics vs. wavelengthfor the optical fibers #2-1, #2-2 and #2-3. All the fibers have adispersion “D” of 2≦D≦8 ps/nm/km in a wavelength band of 1400 to 1700nm, each having an extreme value in the same band. With paying attentionto dispersion slope, the dispersion slope is positive at 1500 nm andnegative at 1550 nm, thereby has an extreme value between 1500 nm and1550 nm. Meantime, a cable cutoff wavelength was; 1550 nm or shorter,and a bending loss was 5 dB/m or less in a bending diameter of 20 mm at1550 nm. As shown in FIG. 9, a zero-dispersion wavelength λo was 1350 nmor shorter, where the Raman amplification band could be expanded to theS-band. Moreover, an effective area was maintained to 40 μmm² or more(preferably 45 μmm² or more), thereby suppressing non-linearity as inthe conventional DSF.

As shown in FIG. 9, each fiber according to the second embodiment has atleast an extreme value of the dispersion in the given wavelength band,which refers to neither monotonous increase nor monotonous decrease,suppressing the FWM and providing a broader wavelength band of adesirable level of 2-8 ps/nm/km (see Table 2), compared to theconventional optical fiber #2-4: reduced dispersion slope NZ-DSF,dispersion slope of 0.020 ps/nm²/km at 1550 nm, and #2-5: True Wave(registered trademark) RS, dispersion slope of 0.045 ps/nm²/km at 1550nm).

Here, a dispersion is 2-8 ps/nm/km in the wavelength band of 1400 nm to1700 nm and can suppress FWM and cumulative dispersion in itself, whichenables the transmission without the aid of dispersion compensation. Theconventional NZ-DSF, proposed so far, enables the transmission at mostin the S-, C- and L-bands (1460-1625 nm, see #2-4 in FIG. 9). Thepresent NZ-DSF has dispersion characteristics enabling transmission inthe U-band (1625-1675 nm) and E-band (1360-1460 nm) in addition to theabove three bands. At present, the U-band transmission are limited bybending loss and UV absorption loss increases, while the E-bandtransmission is limited mainly by hydroxyl ion loss at 1385 nm orthereabout. From this perspective, the present NZ-DSF has a moderatedispersion applicable for the U- and E-band, with breaking through theabove two limitations, which does not require a need of a dispersioncompensator even in the U-band. Also, featuring is a relatively flatdispersion over the wavelength bands. Especially, the #2-3 in FIG. 9 hasa dispersion difference of not more than 1 ps/nm/km (0.7 ps/nm/km orless) in the S-, C- and L-bands (1460-1625 nm), which shows a most flatdispersion in a broad wavelength band.

Even in the case of applying the present optical fiber to an opticaltransmission system with a distributed Raman amplification, thedispersion of the present fiber is 2 ps/nm/km or more at the shortestwavelength of the Raman pumping band (1350-1360 nm), which prevents theFWM generation That is, the present optical fiber has a moderatedispersion of around 2 ps/nm/km at 1350-1360 nm which corresponds to thepump light wavelength for the shortest wavelength of 1460 nm in theS-band. The dispersion of the present fiber in the Raman pumpingwavelength band for the C or L band transmission is, of course, 2ps/nm/km or more and #2-1, #2-2 and #2-3 in Table 2 can be applied tothe distributed Raman amplification in the S. C and L-bands, consideringtheir dispersion curves.

FIG. 14 and FIG. 15 show an optical communication systems with anoptical transmission line using the second embodiment optical fibers.Detail description of these drawings is mentioned later.

As shown in the above, the dispersion characteristics of optical fibersaccording to the second embodiment can allow distributed Ramanamplification in the S, C and L-bands simultaneously. However,simultaneous Raman amplification over the S, C and L-bands can hardly bemade workable, because the pumping light wavelengths overlap thetransmission light wavelengths. This problem can be solved, by providinga distributed Raman amplifier for the S- and C-bands and a discreteRaman amplifier or an EDFA for the L-band, for example.

Embodiment 3

Hereinafter, the third embodiment of this invention is described withreference to the drawings.

FIG. 10( a) shows refractive index profiles and FIG. 10( b) shows across-section in the third embodiment of an optical fiber according tothe present invention. Various types of the refractive index profilesare available for optical fibers. Yet, the third embodiment applies arefractive index profile, as shown in FIG. 10( a).

An optical fiber with a refractive index profile as shown in FIG. 10( a)has a core 11 at the center and a cladding 7 surrounding the core 11. Inaddition the core has at least four layers (first 1, second 2, third 3,forth 4 from center to outside), and cladding 7, where any two adjacentlayers differ in composition over each other. The layers are in acoaxial layout. The cladding serves as the reference refractive indexindicator for the four layers.

The relative refractive index differences of the first, second, thirdand forth layers (given Δ1, Δ2, Δ3 and Δ4) with reference to cladding 7are chosen such that Δ1>Δ4>Δ3>Δ2, where any adjacent two layers differin refractive index.

In addition, the first layer as shown in FIG. 10, has α-profile with αof 4 or more.

Subsequently, the first layer 1 is made to a relative refractive indexdifference Δ1 of not less than 0.3 and not more than 0.7%,and a ratio ofdiameters of the first layer “a” to the cladding “D” of 0.05 to 0.1. Forexample, in the case with the cladding diameter D of 125 μm, the firstlayer is required to be 6.5 to 12 μm in diameter a.

Meanwhile, the second layer 2 is made to a relative refractive indexdifference Δ2 of not less the −0.6 and not more than −0.2%, and a ratioof diameters of the second layer “b” to the first layer “a” of 1.3 to1.8.

The third layer 3 is made to a relative refractive index difference Δ3of not less than −0.2 and not more than −0.05%, and a ratio of diametersof the third layer “c” to the first layer “a” of 1.9 to 2.4. The forthlayer 4 is made to a relative refractive index difference Δ4 of not lessthan 0.1 and not more than 0.55%, and a ratio of diameters of the fourthlayer “d” to the first layer “a” of 2.6 to 2.8. Here, the above relativerefractive index differences of Δ1 to Δ4 and diameters of “a” to “d” offirst, second third and forth layers in the refractive index profile asshown in FIG. 10 are defined as the average values of horizontal andvertical portions, respectively.

Here, the relative refractive index differences Δ1, Δ2, Δ3 and Δ4,diameters a, b, c and d, and α factor of the optical fiber wasdetermined as shown below. For the optical fiber as shown in FIG. 10,simulation was made, varying the above parameters of relative refractiveindex differences Δ1, Δ2, Δ3 and Δ4, α factor and diameters a, b, c andd. As a result, the dispersion over the S, C and L-bands (1460-1625 nm)fell within a range of 2 to 8 ps/nm/km and a difference between themaximum and minimum dispersion was not more than 4 ps/nm/km. The optimalvalues for Δ1, Δ2, Δ3 and Δ4, a, b, c and d, and α were figured out, andoptimum values at the wavelength of 1550 nm are shown in Table 3.

TABLE 3 a:b:c:d Δ1 Δ2 Δ3 Δ4 α a b c d #3-1 0.47 −0.4 −0.05 0.4 8 1 1.72.1 2.6 #3-2 0.48 −0.45 −0.1 0.4 8 1 1.5 2.1 2.5 #3-3 0.50 −0.45 −0.10.35 8 1 1.5 2.1 2.5 #3-4 0.48 −0.4 −0.05 0.4 6 1 1.6 2.1 2.5 #3-5 0.52−0.4 −0.1 0.4 4 1 1.5 2.1 2.5

The above simulation was made by using subparameters of dispersion at1550 nm, dispersion slope at 1550 nm, effective area at 1550 nm, thecable cutoff wavelength in a length of 22 m, zero-dispersion wavelength,and bending loss in 20 mm diameter at 1550 nm, which change in alongwith the change of relative refractive index differences Δ1, Δ2, Δ3 andΔ4, α factor and diameters a, b, c and d.

Table 4 presents the values of the subparameters which result in thevalue of parameters in Tables 3. The wavelength characteristics of thechromatic dispersion for simulated optical fibers #3-1 to #3-5 areplotted in FIG. 11.

TABLE 4 Dis- Dispersion Bending Core persion slope Aeff λc λ₀ lossdiameter #3-1 6.12 0.0016 46 <1550 1334 3.0 19.3 #3-2 7.56 0.0095 45<1550 1328 2.0 18.8 #3-3 4.41 0.0007 44 <1550 1355 2.0 18.7 #3-4 7.280.0096 47 <1550 1333 1.0 19.7 #3-5 7.74 0.0096 43 <1550 1335 3.0 18.9

Simulation results in Table 4 were obtained when the parameters ofrelative refractive index differences Δ1 to Δ4, α factor and diameters ato d are applied as per Table 3. In all of simulation cases, an opticalfiber had a dispersion of 2 ps/nm/km or more at 1550 nm in the C-band,and a dispersion slope of positive and not more than +0.010 ps/nm²/km, acable cutoff wavelength (λc) of 1550 nm or shorter and a bending loss of5 dB/m or less in 20 mm diameter.

In addition, all of the zero-dispersion wavelengths (λo) in Table 4 are1360 nm or shorter, and the optical fibers in the third embodiment makeit possible to expand the transmission wavelength band, which enablesRoman amplification, to the S-band. As these optical fibers in Table 4each have an effective area of 40 μm² or more, it is expected tosuppress the non-linearity effect when WDM transmission is made at awavelength which allows the same effective area to be available.

Meanwhile, additionally considering FIG. 11, optical fibers conformingto relative refractive index differences Δ1 to Δ4, α factor anddiameters a to d in Table 3, turn out to have a difference between themaximum and minimum dispersion of not more than 2 ps/nm/km throughout awavelength band between 1460 and 1625 nm. In other words, theirdispersion slopes were reduced remarkably, throughout the broad band of1460 nm to 1625 nm. Simulated characteristics of optical fibers in thethird embodiment are a dispersion of 2 to 8 ps/nm/km between 1400 and1650 nm, particularly between 1370 and 1650 nm except #3-3, and it ispossible to realize broad band of flatness in dispersion over awavelength span of 250 nm (a wavelength span of 280 nm except #3-3) andsuppression of the FWM generation.

Now, FIG. 12 presents the Raman pumping wavelength band of 1360 to 1525nm for the transmission wavelength band of 1460 to 1625 nm in additionto FIG. 11, along with the dispersion curve for a optical fiber with adispersion of 2 to 8 ps/nm/km in the Raman pumping wavelength band. FIG.12 shows that as a result of simulation, optical fibers in the thirdembodiment have a zero-dispersion wavelength shorter than the Ramanpumping band, unlike the conventional DSF, making it possible to realizedistributed Raman amplification over the S, C and L-bands (1460 to 1625nm).

The optical fiber according to the third embodiment in the presentinvention has at least four layers (first to forth layers 1 to 4), andis allowed to have five layers or more.

Three types of optical fibers were made on trial, based on thesimulation results in Tables 3 and 4, and measured for dispersion,dispersion slope; effective area, cable cutoff wavelength andzero-dispersion wavelength, each at 1460 nm, 1550 nm and 1620 nm,including bending loss in 20 mm diameter at 1550 nm. Table 5 presentsthe measured results of #5-1, #5-2 and #5-3, and moreover FIG. 13 showstheir dispersion characteristics.

TABLE 5 Dispersion Bending Dispersion slope Aeff λc loss 1460 nm 1550 nm1625 nm 1550 nm 1550 nm 22 m λ₀ 1550 nm #5-1 5.32 7.02 7.07 0.0087 45933 1340 5 #5-2 4.32 5.75 5.51 0.0054 45 925 1352 8 #5-3 5.81 7.71 7.950.0100 45 945 1335 2

As apparent from Table 5 and FIG. 13, trials #5-1, #5-2 and #5-3 weremade so that relative refractive index differences Δ1 to Δ4, α factorand diameters a to d were the above-mentioned values. As a result, theyhad 2 to 8 ps/nm/km in dispersion and at most 0.01 ps/nm²/km indispersion slope in the S, C and L-bands (1460 to 1625 nm), and therebythe difference between the maximum and minimum dispersion of each fiberwas not more than 3 ps/nm/km. Each fiber had a dispersion of not lessthan 2 ps/nm/km in S-band of 1460 to 1530 nm, and thereby it waspossible to suppress the FWM generation. In addition, as shown in Table5, all of the trials #5-1, #5-2 and #5-3 had a cable cutoff wavelengthof not longer than 950 nm, and an effective area (A_(eff)) of not lessthan 40 μm², and thereby it was possible to suppress the non-linearityeffect.

However, the specified parameters in the third embodiment for relativerefractive index differences Δ1 to Δ4, α factor and diameters a to d,are based on simulation results, and therefore might not consistent withthose of actually made optical fibers. For example, the simulationshowed a bending loss of 5 dB/m or less, but the trials had greater onesthan simulated. Therefore, a bending loss might be close to 10 dB/m ifan optical fiber is made so that the above-mentioned parameters forrelative refractive index differences Δ1 to Δ4, α factor and diameters ato d.

In addition, the three trials were estimated for mode field diameter(MFD), transmission loss and polarization mode dispersion (PMD) withtransmission signals at 1550 nm, and the results are shown in Table 6.

TABLE 6 Transmission MFD loss 1550 nm 1550 nm PMD #5-1 7.5 0.218 0.029#5-2 7.5 0.254 0.043 #5-3 7.5 0.207 0.049

Here, trials were made so that relative refractive index differences Δ1to Δ4, a factor and diameters a to d had the above-mentioned values, andthereby had a mode field diameter of 7.5 μm, a transmission loss of0.207 to 0.254 dB/km and a polarization mode dispersion of 0.029 to0.049 ps/√{square root over ( )}km as shown in Table 6. All of thesecharacteristics fell within preferable range, particularly, the PMD isless than 0.10 ps/√{square root over ( )}km. Thereby it is possible toprevent the degradation of the WDM transmission in quality by reducingthe PMD as much as possible. In addition, all of the trials #5-1, #5-2and #5-3, have a zero dispersion wavelength (λo) of 1360 nm or shorter.Consequently in the optical fiber according to the third embodiment, itis possible to expand the transmission band, applicable for Ramanamplification, to the S-band because the problem such as FWM in theRaman pump light band can be prevented.

Thus, trials #5-1, #5-2 and #5-3 demonstrates that in optical fibersaccording to the third embodiment, reduced dispersion slope leads to aflattened wavelength characteristic of dispersion and furthermorelowered cumulative dispersion on the longer wavelength-side. Moreover,it is possible to expand a wavelength band of avoiding FWM occurrence tothe shorter wavelength-side. Consequently, it turns out to realize theWDM transmission in all of the S, C and L-bands.

FIG. 14 shows a diagram for a first example optical transmission systemaccording to the third embodiment. Optical transmission system 310comprises of optical fibers according to the third embodiment as theoptical transmission line, a distributed optical amplifier 311, adiscrete optical amplifiers 317, 318 for amplify at least one of the S,C and L-bands, and a dispersion compensator 320.

In the transmission line, a multiplexer 316 and a demultiplexer 315 arelocated apart from each other at the inlet of the distributed Ramanamplifier 311, and the dispersion compensator 320 is located at theoutlet of the distributed Raman amplifier 311.

Distributed Raman amplifier 311 has a pump light source 312 for theS-band and a demultiplexer 314, and the pump light source 312 isconnected to the demultiplexer 314 through the optical fiber 313 andapplied to the counter-propagation pumping for the S-band transmissionsignals.

Discrete optical amplifiers 317 and 318 are erbium-doped fiberamplifiers (EDFA) connected to demultiplexer 315 and multiplexer 316,through optical fibers 319. The former 317 is provided for the C-bandand the latter 318, for the L-band.

Dispersion compensator 320 has three dispersion compensating fibers 323,325, 327, which interconnect demultiplexer 321 and multiplier 322.Dispersion compensating fibers 323, 325, 327, are provided for S, C andL-bands, respectively, and equipped with optical amplifiers 324, 326,328, to compensate the signal loss in each band

Optical transmission system 310 in the first example comprises ofoptical fibers according to the third embodiment, as the transmissionline, and a distributed Raman amplifier 311. Accordingly, opticaltransmission system 310 makes it possible to reduce the required maximumpower given to the optical fiber, and thereby makes it possible tosuppress signal distortion due to a non-linearity effect in the opticalfiber.

In addition to the above distributed Raman amplifier, a discrete Ramanamplifier is available, but a non-linearity effect cannot be negligibleif the discrete Raman amplifier is applied to a WDM transmission. Yet,optical transmission system 310 in the first example comprises of anoptical fiber having an effective area (A_(eff)) of 40 μmm² or more at1550 nm, and therefore makes it possible to prevent a signal distortiondue to a non-linearity effect in the 1550 nm WDM transmission.

FIG. 15 shows a diagram for a second example optical transmission systemof the third embodiment. Optical transmission system 330 makes pumplight source 312 to serve for counter-propagation pumping, in the C- andS-bands, unlike the pimp light source of 312 in FIG. 14.

Therefore, optical transmission system as shown in FIG. 15 requires onlya discrete optical amplifier for the L-band, without a need of adiscrete optical amplifier for the C-band. Therefore, it is possible toreduce number of optical devices constructing an optical transmissionsystem.

Moreover, the optical fiber according to the third embodiment makes itpossible to reduce a cumulative dispersion, and so it isn't necessary toapply any dispersion compensator 320 as shown in FIG. 14 or FIG. 15, atthe 10 Gps signal transmission. Therefore as shown in FIG. 16, it ispossible to even reduce number of optical devices. Similarly, opticalfibers according to the third embodiment have a least dispersiondifference throughout the band, and hence makes it possible to apply theconventional dispersion compensator designed for the standardsingle-mode optical fiber at high speed transmission of 40 Gbps or more,and thereby has an advantage of not requiring a new design of adispersion compensator.

Embodiment 4

Hereinafter, the fourth embodiment of this invention is described withreference to the drawings.

FIG. 17( a) shows a refractive index profile and FIG. 17( b) shows across-section of an optical fiber according to the fourth embodiment ofthe present invention. The refractive index profile in FIG. 17( a)features to be relatively simple and easy to design and control,compared to the other profiles.

Optical fiber comprises a core 11 at the center and a cladding 7surrounding the core 11. In addition, the core has at least three layersfirst 1, second 2, and third 3 from center to outside), and cladding 7with the reference refractive index, where any two adjacent layersdiffer in composition over each other The layers are in a coaxiallayout. The cladding serves as the reference refractive index indicatorfor the three layers.

In addition, relative refractive index differences of the first layer 1and the third layer 3 with the cladding 7 are made positive, and that ofthe second layer 2 is made negative. First layer 1 has an α-profile.

In the optical fiber having three layered core according to the forthembodiment, the three relative refractive indices of the core are chosensuch that Δ1>Δ3>0>Δ2 (given Δ1 for the first layer, Δ2 for the second,Δ3 for the third, 0 for the cladding).

Here, “a” denotes a diameter of first layer 1, “b” denotes a diameter ofsecond layer 2, “c” denotes a diameter of third layer 3. The parametersof Δ1, Δ2 and Δ3 in relative refractive index and a, b and c indiameters of each layers which are applied in the fourth embodiment arenot limited to a particular value, but preferably over the followingrange.

That is the preferred ranges of relative refractive index differencesare: 0.3 to 0.8% for Δ1, −0.6 to 0.05% for Δ2, and 0.05 to 0.4% for Δ3.Also, the preferred ratio of diameters are: 1.5 to 2.2 for b/a and 2.2to 3.5 for c/a.

An optical fiber in the fourth embodiment has a dispersion of 4 ps/nm/kmor more at 1550 nm, and a positive dispersion slope of 0.050 ps/nm²/kmor less (preferably positive and not more than 0.025 ps/nm²/km) at leastat a predetermined wavelength within a range of 1460 nm to 1625 nm.

An optical fiber in the fourth embodiment has a cutoff wavelength of1550 nm or shorter (preferably 1450 nm or shorter) at a length of 2 m, azero-dispersion wavelength of 1460 nm or shorter (preferably 1400 nm orshorter) and a transmission loss of 1.5 dB/km or less at 1385 nm.

An optical fiber in the fourth embodiment has an effective area of 40 to60 μm² (preferably 45 μmm² or less) in a predetermined wavelength withina wavelength range of 1460 nm to 1625 nm, and bending loss of 5 dB/m orless in a bending diameter of 20 mm in at 1550 nm and a polarizationmode dispersion of 0.08 ps/√{square root over ( )}km or less at 1550 nm.

The present inventor, at first, made studies to determine theconstruction of optical fibers in the fourth embodiment, applicable forthe WDM transmission mainly in the C-band.

As in general in the Raman amplification for the C-band, a pump lightwavelength is 1420-1430 nm for the shortest wavelength of 1530 nm and apump light wavelength is 1455-1465 nm for the longest wavelength of 1565nm. Here a difference of the pump light power would be preferably within±10% over the wavelength band.

In other words, should the pump power fall within ±10% difference, allthe WDM transmission signals can be adjusted to a required gain, simplyby an intensity-adjustment of the each pump light source. On the otherhand, more than ±10% of loss difference would additionally require theadjustment of the wavelength spacing of plurality of pump light sources.As a consequence, it prevents to determine the wavelength spacing ofpump light sources in advance and also results in increasing number ofoptical devices, which causes a disadvantage in cost.

Curve “a” in FIG. 18 represents transmission loss at 1385 nm (absorptionloss arising from hydroxyl ion) vs. Loss difference at 1420 to 1430 nm.Also, curve “b” shows transmission loss at 1385 nm vs. Loss differenceat 1455 to 1465 nm. Here, the loss difference is defined as a differenceof loss at an interval of 10 nm of wavelength in each wavelength hand.

As apparent from Curves “a” and “b”, the transmission loss of not morethan 1.5 dB/km, at 1385 nm should be made, so as to fall within ±10% ofthe transmission loss difference at an interval of 10 nm wavelength.Considering the wavelength dependence of transmission loss, thetransmission loss difference should be suppressed to be ±10% or lessover the wavelength range of 1420 to 1430 nm and 1455 to 1465 nm.

FIG. 19( a) plots a characteristic curve: wavelength dependence oftransmission loss, for an optical fiber with a transmission loss of notmore than 1.5 dB/km at 1385 nm; FIG. 19( b) plots a characteristiccurve: wavelength dependence of transmission loss for an optical fiber,with a transmission loss of more than 1.5 dB/km at 1385 nm.

FIG. 20 plots a relationship between transmission loss at 1385 nm andhydrofluoric acid etching thickness of the surface of optical fiberpreform, whose first, second, and third layer are manufactured by VADmethod, conforming to FIG. 17. Consequently, the removal of 1 mm fromthe surface can result in less than 1.5 dB/km of transmission lossincrease at 1385 nm.

Accordingly, an optical fiber in the fourth embodiment which was etchedmore than 1 mm from the surface, turned out to have a transmission lossless than 1.5 dB/m at 1385 nm, making the transmission loss to fallwithin ±10% of loss difference in the pump light wavelength band.

The Raman amplification gain is inversely proportional to the effectivearea, and so considerably decreased by an extremely large effectivearea. Therefore, an optical fiber in the fourth embodiment was adjustedto an effective area of 60 μmm² or less at least at a predeterminedwavelength within a wavelength range of 1460 nm to 1625 nm. Opticalfibers in the fourth embodiment can suppress the reduction of Ramanamplification efficiency both in distributed and discrete Ramanamplifiers.

In addition, an extremely small effective area may cause signaldistortion due to a non-linearity effect of self phase modulation (SPM)and cross phase modulation (XPM) etc. Then, the fourth embodimentapplied an effective area of not less than 40 μmm² to suppress signaldistortion due to SPM or XPM.

Also, a least dispersion at 1550 nm in the C band may lead into the FWMgeneration. Then, the fourth embodiment applied a dispersion of not lessthan 4 ps/nm/km at 1550 nm to suppress signal distortion due to FWMgeneration.

An optical fiber in the fourth embodiment was made to have a cutoffwavelength of 1550 nm or shorter to ensure single-mode transmission inthe C-band.

For reference, shorter the cutoff wavelength, the better in transmissionperformance. It is desirable to apply a cutoff wavelength of 1450 nm orshorter, considering transmission systems with the Raman amplification.

As for WDM transmission, the increase in dispersion slope will result inthe large dispersion difference over the wavelength band, bringing anadverse effect on high capacity and high speed transmission. That is,WDM transmission requires each of the optical fibers with a reduceddispersion slope to suppress the dispersion difference between thewavelength band.

As a result, an optical fiber in the fourth embodiment has a dispersionslope of positive and 0.050 ps/nm²/km or less at least in apredetermined wavelength within a wavelength range of 1460 nm to 1625nm. Also, an optical fiber in the fourth embodiment is made to have azero-dispersion wavelength of 1460 nm or shorter Then, optical fibers inthe fourth embodiment can suppress signal distortion due to dispersionbecause of suppressed difference in dispersion over the wavelength band.

Moreover, an optical fiber in the fourth embodiment is preferably madeto have a dispersion slope of positive and 0.025 ps/nm²/km or less atleast in a predetermined wavelength within a wavelength range of 1460 nmto 1625 nm, and a zero-dispersion wavelength of 1400 nm or shorter,considering the above mentioned dispersion slope vs. dispersioncharacteristics. Therefore, the optical fiber in the fourth embodimentrealizes to suppress the dispersion difference and signal distortion dueto dispersion more than ever.

In addition, an optical fiber having a large bending loss adverselyaffect when arranged in a module. However, an optical fiber in thefourth embodiment does not exceed 5 dB/m of bending loss in a bendingdiameter of 20 mm at 1550 nm. An optical fiber in the fourth embodiment,for example, enables coiling to be arranged in a module, with a leastloss increase. Generally, the less in bending loss, the more reliable inperformance.

Similarly, a large polarization-mode dispersion (PMD) leads into aconsiderable signal delay time in high-speed transmission. An opticalfiber in the fourth embodiment has not more than 0.08 ps/√{square rootover ( )}km at 1550 nm to be free from the PMD interference, andsuitable for the Raman amplification.

Meanwhile, the present inventor has optimized the refractive indexprofiles in view of the above specified parameters and characteristics,as follows:

In short, concerning the refractive index profile in FIG. 17, it turnedout that, when the relative refractive index difference of first layer 1(Δ1), is made greater than 0.8%, a dispersion slope of positive and notmore than 0.05 ps/nm²/km and an effective area of 40 μmm² or more couldnot be rented; and at the same time that less than 0.3% of relativerefractive index difference Δ1, would cause more than 5 dB/m of bendingloss. As a result, the relative refractive index difference Δ1 wasdetermined to be in a range between 0.3% and 0.8%.

Moreover, within the Δ1 range between 0.3% and 0.8%, an optimized valueof “α” which allows an increase in the effective area, withoutincreasing the dispersion slope, was found to be 4 or more. Similarly,more than −0.05% of relative refractive index difference of second layer2 (Δ2) resulted in more than 0.05 ps/nm²/km of dispersion slope, whileless than −0.6% of relative refractive index difference (Δ2) resulted inless than 40 μmm² of effective area. As a result, the relativerefractive index difference Δ2 was determined to a range between −0.6%and −0.05%.

When diameter “b” of second layer 2 is made to be more than 2.2 timesdiameter “a” of first layer 1, the dispersion slope exceeds 0.05ps/nm²/km. In contrast, when diameter “b” of second layer 2 is made toless than 1.5 times diameter “a” of first layer, the effective areafalls less than 40 μmm². As a result, an appropriate value of “b/a” wasdetermined to be in a range between 1.5 and 2.2.

Similarly, more than 0.4% of relative refractive index difference ofthird layer Δ3 results in longer than 1550 nm of cutoff wavelength,while less than 0.05% of relative refractive index difference Δ3 resultsin higher than 0.05 ps/nm²/km of dispersion slope As a result, therelative refractive index difference Δ3 was determined to be in a rangebetween 0.0.5% and 0.4%.

Furthermore, when diameter “c” of third layer 3 is made to more than 3.5times diameter “a” of first layer 1, the cutoff wavelength exceeds 1550nm. Diameter “c” of third layer 3 is made to less than 2.2 timesdiameter “a” of first layer 1, the dispersion slope exceeds 0.05ps/nm²/km or the effective area falls less than 40 μm². As a result, aoptimized value of “c/a” was determined to be in a range between 2.2 and3.5.

The fourth embodiment has such excellent characteristics, and providesthe optical fiber suitable for the WDM transmission with the Ramanamplification in the C-band, by optimizing the refractive indexprofiles, as mentioned above.

In addition, optical transmission system applying optical fibers in thefourth embodiment as an optical transmission line provides high-qualityWDM transmission with Raman amplifiers.

Table 7 presents optical fibers made on trial according to the forthembodiment.

TABLE 7 Dispersion Transmission Bending Dispersion slope MFD Aeff lossloss 1460 nm 1550 nm 1620 nm 1550 nm 1550 nm 1550 nm 1385 nm 1550 nm λcλ₀ PMD 1550 nm #7-1 — 4.8 — 0.0292 7.8 45.2 1.185 0.209 1159 1412 0.0450.212 #7-2 — 5.6 — 0.0293 8.0 47.8 0.893 0.207 1446 1393 0.048 0.021#7-3 — 4.6 — 0.0444 8.5 54.6 0.658 0.202 1070 1448 0.030 4.300 #7-4 —7.7 — 0.0226 7.6 43.8 1.457 0.211 1101 1353 0.053 3.210 #7-5 — 8.8 —0.0325 7.7 44.4 0.303 0.193 1222 1350 0.057 0.041 #7-6 2.0 4.8 6.60.0252 7.8 45.2 1.185 0.209 1159 1402 0.045 0.212 #7-7 2.2 4.2 5.00.0156 7.5 42.6 1.411 0.206 1156 1396 0.026 4.018 #7-8 2.7 4.9 6.10.0200 7.6 44.5 0.983 0.211 1273 1387 0.027 0.914

Each dispersion value is with a unit of ps/nmkm and each dispersionslope with ps/nm²/km. Measured wavelengths are shown above the columnsof measurements.

Each MFD is mode filed diameter (μm), Aeff is effective area (μm²), Lossis transmission loss (dB/km), Bend loss is bending loss in 20 mmdiameter (dB/m), λc is cutoff wavelength (nm), λ0: is zero-dispersionwavelength (nm), and PMD is polarization mode dispersion (ps/√{squareroot over ( )}km)

Curve “a” in FIG. 21 presents the wavelength dependence of dispersionfor optical-fiber trial #7-8 in Table 8. Curves “b” and “c” (wavelengthdependence of dispersion) resulted from optical fibers with 0.045 and0.060 ps/nm²/km of dispersion slope, respectively. Each was with 4.9ps/nm/km of dispersion.

As shown in curve “a” in FIG. 21, optical fiber trial #7-8 has adispersion of +2 to +8 ps/nm/km over a broad wavelength band of the S-,C-, and L-band (1460 to 1625 nm). Then, optical-fiber trial #7-8 cansuppress the FWM generation in these bands to carry out high-quality WDMtransmission.

Thus, a dispersion of not less than 4 ps/nm/km at 1550 nm, and adispersion slope of positive and not more than 0.025 ps/nm²/km at apredetermined wavelength within a wavelength range of 1460 nm to 1625nm, can realize a dispersion of 2 to +8 ps/m/km over a broad range fromthe S-band to L-band. Therefore, optical-fiber trial #7-8 can suppressthe FWM generation in these bands to carry out high-quality WDMtransmission.

Curve “c” in FIG. 21 resulted from an optical fiber having the samedispersion as #7-8, which had a dispersion slope of 0.060 ps/nm²/km,around three times as large as that of #7-8. The rather high slope canmake the dispersion too low in the S-band, inducing the FWM generationConversely, the dispersion can make the dispersion too large in theL-band, allowing signal distortion due to dispersion.

In addition, Curve “b” in FIG. 21 resulted from an optical fiber, whichhad a dispersion slope of 0.045 ps/nm²/km, somewhat larger than that of#7-8. The dispersion is expected to be slightly large in the L-band, butto fall to 2 ps/nm/km or less in the S-band, with more likely generationof the FWM Notably, curve “b” is even more preferable than curve “c” indispersion.

Curves “a”, “b” and “c” in FIG. 22 are wavelength dependencecharacteristics in dispersion of optical fiber trials (#7-6, #7-7 and#7-8) in Table 7. #7-6 and #7-7, similarly with #7-8, falls with adispersion range of 2 to +8 ps/nm/km widely from the S-band to L-band,suppressing FWM interference or dispersion caused signal distortion andrealizing high-quality WDM transmission.

Finally, the fourth embodiment cannot be limited to the examples whichare shown and described in this specification, and can be embodied inother various construction. One of the above embodied optical fibers hasa three-layered core of first second and third layers. But, a more thanthree-layered core can be designed for an optical fiber: a four-layeredcore, a five-layered core, and so on.

Meanwhile, optical fibers according to the fourth embodiment wouldpreferably conform to the required values for bending loss, dispersionor transmission loss in the above embodiments, but may deviate more orless from them.

1. An optical fiber comprising a core and cladding, characterized by: adispersion “D” of not less than 2 ps/nm/km and not more than 8 ps/nm/km(2≦D≦8) over a wavelength range of 1400 to 1700 nm and, at least oneextreme value of dispersion within said wavelength range, the opticalfiber further comprising: a plurality of glass layers inside of thecladding having a reference refractive index, wherein one of twoadjacent layers differs in composition from the other, first, second,third and fourth layers from the center to outside have relativerefractive index differences of Δ1, Δ2, Δ3 and Δ4 from said referencerefractive index of said cladding (0), respectively, and satisfy thefollowing condition, Δ1>Δ3>Δ4>0Δ2.
 2. The optical fiber of claim 1,which is further characterized by: a dispersion “D” of not less than −4ps/nm/km and not more than 4 ps/nm/km (−4≦D≦4) at 1310 nm.
 3. Theoptical fiber of claim 1, which is further characterized by: a cablecutoff wavelength of 1550 nm or shorter.
 4. The optical fiber of claim1, which is further characterized by: a bending loss of 5 dB/m or lessin a bending diameter of 20 mm at 1550 nm.
 5. The optical fiber of claim1, which is further characterized by: an effective area (Aeff) of 40 μm²or more at 1550 nm.
 6. The optical fiber of claim 1, which is furthercharacterized by: a polarization mode dispersion (PMD) of 0.1 ps/√km orless at 1550 nm.
 7. The optical fiber of claim 1, which is furthercharacterized by: a dispersion “D” of not less than −2 ps/nm/km and notmore than 2 ps/nm/km (−2≦D≦2) at 1310 nm.
 8. The optical fiber of claim1, which is further characterized by: a zero dispersion wavelength of1350 nm or shorter.
 9. The optical fiber of claim 1, wherein: themaximum of said relative refractive index difference of said first layerΔ1 with said cladding is 0.3 to 0.7%, α factor of said first layer is 4or more, the diameter of said cladding diameter is 124 to 126 μm, thediameter of said first layer is 7.0 to 10 μm.
 10. The optical fiber ofclaim 1, wherein: said relative refractive index difference of saidsecond layer Δ2 with said cladding is −0.6 to −0.2%, a ratio ofdiameters of said second layer to said first layer is 1.2 to 1.8, saidrelative refractive index difference of said third layer Δ3 with saidcladding is 0.25 to 0.5%, a ratio of diameters of said third layer tosaid first layer is 1.8 to 2.2, said relative refractive indexdifference of said fourth layer Δ4 with said cladding is 0.05 to 0.2%, aratio of diameters of said fourth layer to said first layer is 2.0 to2.7.
 11. An optical fiber comprising a core and cladding, characterizedby: a dispersion “D” of not less than 2 ps/nm/km and not more than 8ps/nm/km (2≦D≦8) over a wavelength range of 1400 to 1700 nm, and atleast one extreme value of dispersion within said wavelength range,wherein one of any two adjacent layers differs in composition from theother, first, second ,third, fourth and fifth layers from center tooutside have relative refractive index differences, of Δ1, Δ2, Δ3,Δ4 andΔ5 with said reference refractive index of said cladding (0),respectively, and satisfy the following condition, Δ1>Δ4>Δ5>0>Δ3>Δ2. 12.The optical fiber of claim 11, wherein: the maximum of said relativerefractive index difference (Δ1) of said first layer with said claddingis 0.3 to 0.7%, α factor of said first layer is 4 or more, said claddingdiameter is 124 to 126 μm, the diameter of said first layer is 6.5 to 10μm.
 13. The optical fiber of claim 11, wherein: said relative refractiveindex difference of said second layer Δ2 with said cladding is −0.6 to−0.2%, a ratio of diameters of said second layer to said first layer is1.2 to 1.8, said relative refractive index difference of said thirdlayer Δ3 with said cladding is −0.15 to 0.05%, a ratio of diameters ofsaid third layer to said first layer is 1.8 to 2.2, said relativerefractive index difference of said fourth layer Δ4 with said claddingis 0.25 to 0.65%, a ratio of diameters of said fourth layer to saidfirst layer is 2.0 to 2.7, said relative refractive index difference ofsaid fifth layer Δ5 with said cladding is 0.05 to 0.50%, a ratio ofdiameters of said fifth layer to said first layer is 2.2 to 3.0.
 14. Anoptical communications system comprising the optical fibers of claim 1and a distributed Raman amplifier for the S-band.
 15. The optical fiberof claim 11, which is further characterized by: a dispersion “D” of notless than −4 ps/nm/km and not more than 4 ps/nm/km (−4≦D≦4) at 1310 nm.16. The optical fiber of claim 11, which is further characterized by: acable cutoff wavelength of 1550 nm or shorter.
 17. The optical fiber ofclaim 11, which is further characterized by: a bending loss of 5 dB/m orless in a bending diameter of 20 mm at 1550 nm.
 18. The optical fiber ofclaim 11, which is further characterized by: an effective area (Aeff) of40 μm² or more at 1550 nm.
 19. The optical fiber of claim 11, which isfurther characterized by: a polarization mode dispersion (PMD) of 0.1ps/√km or less at 1550 nm.
 20. The optical fiber of claim 11, which isfurther characterized by: a dispersion “D” of not less than −2 ps/nm/kmand not more than 2 ps/nm/km (−2≦D≦2) at 1310 nm.
 21. The optical fiberof claim 11, which is further characterized by: a zero dispersionwavelength of 1350 nm or shorter.
 22. An optical communications systemcomprising the optical fibers of claim 11 and a distributed Ramanamplifier for the S-band.